Both coplanar waveguide (CPW) and grounded coplanar waveguide (CPWG) are ideal (superior to most commonly used microstrip) for surface mounting components whose performance benefits from minimal ground inductance and good thermal conductivity. These are the "virtues" of utilizing CPW or CPWG. A good example of such a device is a typical gallium arsenide field effect transistor (GaAs FET) for a low noise amplifier (LNA). Such high performance FETs have very high gain and are challenging if not impossible to keep stable unless the device's leads are connected to ground with minimal inductance. A power FET is another example of a typical device that would benefit from the excellent thermal characteristics of CPW. Mounting of such a device with the addition of matching and bias circuitry results in what may be referred to as a single ended CPW or CPWG component.
Often it is desirable to place two single ended components between two 90.degree. 3dB couplers for improved return loss or higher power performance. This "balanced" design often increases the circuit complexity and cost. The addition of couplers also introduces loss at the "front end" and "back end" of the design. Coupler loss is a direct detriment to the noise figure, output power, and gain performance of the design.
One conventional implementation of the 90.degree. couplers on PWB (printed wiring board or PCB printed circuit board) is a microstrip branch line coupler. The branch line coupler is more narrow band than the CPW coupler limiting the usefulness of the branch line coupler. One could also consider using a microstrip lange coupler. However, microstrip lange couplers require fabrication tolerances not possible using standard PWB fabrication techniques. Any such microstrip couplers would require a microstrip to CPW transition at both the input and output of the single ended component thus introducing additional loss. Packaged 90.degree. couplers may also be purchased which may be soldered down to PWB. These couplers may be implemented a number of ways within their package. Most likely they are constructed by means of lumped or broadside coupled striplines.
An interdigitated coplanar waveguide coupler works as described by Bastida & Fanelli in Electronics Letters, Aug. 14, 1980, Volume 16, No. 17, pages 645-646. The CPW coupler described in the article features a single plane of metalization on one surface of either an alumina oxide or a gallium arsenide substrate. Bond wires are used to interconnect the coupler traces and to connect the coupler to the transmission lines. Attachment of the bond wires requires a delicate bonding step. Furthermore, the bond wires are subject to mechanical damage and once bonded, great care must be taken to avoid their damage. Bond wires, because of their small cross sectional area, are inductive which further limits the high frequency performance of the coupler. Prior art couplers are relatively expensive to manufacture.